The invention proceeds from a method for generating a homogeneous mixture for auto-ignition internal combustion engines and for controlling the combustion process.
During the combustion of a homogeneous fuel/air mixture, as a result of decentral activation a parallel release of energy is achieved, in contrast to conventional processes, in which, as a result of central activation by means of an ignition source (Otto process) or by means of injection (Diesel process), a serial combustion of the charge, with a gradually propagating flame front, occurs. Particular advantages of homogeneous combustion as a result of decentral activation of the charge arise from the substantially lower temperature level, that is local peak temperatures are avoided. This results in lower NOx emissions. Due to the homogeneous distribution of the fuel, the soot emissions fall, and as a result of the complete oxidation of C into CO2, advantages in terms of fuel consumption can be achieved.
It is therefore desirable, when a mixture of fuel and air is formed, to have a homogeneous mixture for the purpose of a combustion process which is optimized with regard to exhaust gas and fuel consumption. In diesel internal combustion engines, combustion ideally occurs by virtue of the auto-ignition of a more or less homogeneous mixture of diesel fuel and air. Attempts have been made to perform this homogeneous mixture formation both in the suction pipe and in the combustion space. However, problems have been the high boiling temperature and wide boiling range of diesel fuel, so that, for example in compression-stroke injection or suction pipe injection, with otherwise unchanged control times of the inlet and outlet valves, the time for the evaporation of all the components of the diesel fuel is too short and the temperatures are too low. Moreover, ignition is too early because of the high cetane number of diesel fuel and therefore does not have optimal efficiency. Another problem is a rapid reaction of the homogeneous mixture and the accompanying high pressure rise.
It is an object of the present invention is based is to provide a method for generating a homogeneous mixture for auto-ignition internal combustion engines and for controlling the combustion process, in which an uncontrolled and too early ignition of the mixture is prevented.
The combination according to the present invention of exhaust-gas recirculation or exhaust-gas retention with injection of fuel into the recirculated or retained exhaust gas and subsequent cooling of the fuel/air mixture yields two parameters for controlling the combustion process, in order to achieve an optimal position of the combustion center of gravity and smaller pressure rises.
On the one hand, fuel is injected into the retained or recirculated hot exhaust gas of the preceding combustion cycle, in order to obtain a homogeneous fuel/air mixture. In this case, the quantity and temperature or the energy of the retained or recirculated exhaust gas constitutes an essential correcting variable, by means of which the combustion process, in particular the start and duration of reaction/combustion or the center of gravity of the charge, can be controlled and regulated.
On the other hand, the retained or recirculated exhaust gas is cooled, in order to prevent premature ignition. Cooling lowers the final compression temperature and, by freezing the prereactions, prevents uncontrolled premature ignition of the homogeneous charge. The cooled, recirculated or retained exhaust gas acts as an inhibitor and thus ensures smooth combustion. Furthermore, the combustion center of gravity is shifted to the rear for optimal efficiency. Load regulation can then be performed by a variation in the injected fuel quantity.
According to one embodiment of the present invention, the cooling of the homogeneous fuel/air mixture occurs as a result of expansion cooling by means of late inlet-valve opening or early inlet-valve closing (Miller method). If, for example, the inlet valve is closed relatively early, the cylinder fills up with fresh charge only incompletely, and the charge in the cylinder expands during the intake stroke and cools even before the compression stroke.
In a further embodiment of the present invention, at least part of the exhaust gas of a dispenser cylinder of the internal combustion engine is retained or recirculated, in order to inject fuel into this exhaust gas in order to generate a homogeneous fuel/air mixture both for the dispenser cylinder and for the remaining cylinders of the internal combustion engine, the homogeneous fuel/air mixture being cooled before being fed into a common intake device of the dispenser cylinder and the remaining cylinders, preferably by means of a cooling device which operates according to the compressor refrigerating process, the adsorption refrigerating process, the gas refrigerating process, the steam-jet refrigerating process, an electro thermal method or a combination of the processes mentioned.
The auto-ignition internal combustion engine may be provided with an exhaust-gas turbocharger device, and the intake device may have a compressor and also a charge-air cooler downstream of the latter and a downstream expansion turbine, the homogeneous fuel/air mixture being fed into the intake device selectively at one or more connecting points, of which one connecting point is arranged upstream of the compressor, one connecting point between the compressor and the charge-air cooler, one connecting point between the charge-air cooler and the expansion turbine and one further connecting point between the expansion turbine and an intake manifold of the internal combustion engine.
By the homogenized and precooled fuel/air mixture being recirculated into the intake device, it is thoroughly mixed with fresh intake gas. The subsequent further cooling of the fresh gas mixed with homogeneous fuel/air mixture by means of the charge-air cooler and/or the expansion turbine (turbo-cooling) ensures a substantially higher total intake mass, with the result that the thermal throttling caused by the higher temperature of the fuel/air mixture recirculated into the intake device and therefore power losses are reduced. Due to the lower final compression temperature which is possible by means of turbo-cooling, the auto-ignition of the homogeneous mixture does not occur prematurely, but may be adjusted, by the turbo-cooling parameters being regulated, so that it occurs with optimal efficiency in the region of ignition top dead center.